SEASON 5, EPISODE 521 -
Still feeling responsible for Don's near-fatal stabbing, Charlie puts his focus into tracking down an unnoticed serial killer, with the agents revisiting math theories from when Charlie first started working with the team to help solve the case. John Rubinstein ("Desperate Housewives," "CSI: Crime Scene Investigation") Guest Stars As Gene Evans, an Amateur Sleuth Who Helps Charlie's Investigation and Josh Gad Reprises His Role as Conspiracy Theorist Roy McGill, And Dan Martin Returns As Detective Cates.